1. Field of the Invention
The disclosures herein generally relate to a heater control apparatus, an image forming apparatus, a heater control method, and a program for controlling the lighting of a heater.
2. Description of the Related Art
In an image forming apparatus that uses a fuser unit to heat and fuse a toner image transferred onto a record member such as a paper sheet, the temperature of the fuser unit is properly controlled in consideration of user convenience and energy efficiency. The fuser unit includes a heating roller having an embedded heater such as a halogen heater and a temperature sensor to detect the surface temperature of the heating roller. A heater control apparatus controls the power activation of the heater based on the surface temperature in light of a target temperature to achieve a preset temperature. As a control method, a wave-number-based control scheme is known to those skilled in the art. The wave-number-based control scheme controls the on/off state of a switching element for each half cycle of an alternating current by using a zero-cross signal of a commercial power supply as a reference (see Patent Document 1, for example). Patent Document discloses a heating apparatus that sub-samples waves at constant intervals for application of electric power in response to the surface temperature of the heating roller.
As a fuser unit, a heater of a large electric power exceeding 1000 Watts is generally used in order to shorten the time required to increase surface temperature to a preset temperature upon receiving a print request. Such a high power heater is known to create a rush current at the initial stage of temperature control. Halogen heaters, especially, tend to exhibit a large rush current at the initial stage of temperature control in a low-temperature state because halogen heaters exhibit a low resistance in a low-temperature state. A rush current reduces an alternating current voltage used as a power source, which causes a fluorescent light or the like using the same power source to create a flicker.
At the initial stage of temperature control, thus, a phase control (i.e., soft-start control) scheme may be employed in which a heater is powered on only with part of half a wave, followed by gradually increasing the power-on period (see Patent Document 2, for example). Patent Document 2 discloses an apparatus for controlling the heater of a fuser unit which performs phase control in the state in which a flicker is likely to occur, and performs wave-number-based control in the remaining states.
A frequency for which human vision is sensitive and thus perceives flickers is in the range centered at 8.8 Hz and extending to approximately 10 Hz. When a voltage fluctuation created by wave-number-based control includes a frequency component in the range of 8.8 to 10 Hz, human vision is likely to perceive a flicker.
The heater control apparatus disclosed in Patent Document 1 sub-samples one to three half waves at the initial stage of temperature control in which a rush current may occur. It has turned out that successive application of a control current having such a waveform creates a rush current at the cycle that is likely to cause a flickering sensation.
FIG. 15 illustrates an example of a lighting pattern for which a flicker is likely to be perceived. The illustrated lighting pattern includes frequency components in the range of 8.8 to 10 Hz, and is likely to cause a flicker to be perceived by human vision according to experiments. Accordingly, the occurrence of flickers may be reduced, but it is difficult to suppress a flicker that is perceived by human vision, unless a deliberate measure is taken to remove frequency components that cause a flickering sensation.
In an attempt to prevent a flicker by use of wave-number-based control without using phase control, it is difficult to remove frequency components that cause flickers if the control cycle is fixed as in the case of the heater control apparatus disclosed in Patent Document 1.
Accordingly, it may be preferable to provide a heater control apparatus, an image forming apparatus, a heater control method, and a program that can suppress trouble caused by rush currents without using phase control.